In the evolution of commercial fabrication of semiconductor wafers, larger and larger wafers are processed in bigger and bigger batches. Such processing has pushed the performance envelope of processing equipment, as well as that of the wafer handling and carrying mechanisms needed to move, transport, and retain the wafers during processing.
In many chemical and thermal processing operations, it is often necessary to hold the wafers in precise positions during various processing steps. Relatively large and complex structures such as "boats" or "towers" are typically employed to that end. One example of such a structure is described in U.S. Pat. No. 5,492,229 to Tanaka et al. The Tanaka et al. patent is directed to a vertical boat for holding a plurality of semiconductor wafers. The boat includes two end members and a plurality of support members. In one embodiment, the support members are formed from pipe members cut vertically to provide a long plate member having a cross section of a quarter-circlular arc. In another embodiment, the support members are formed from pipe members cut vertically to provide a long plate member having a cross section of a semicircular arc. The Tanaka et al. patent lists as potential materials for its boats the following: silica glass, silicon carbide, carbon, monocrystal silicon, polycrystal silicon, and silicon carbide impregnated with silicon. The various components are to be welded together if made from silica glass; otherwise, "they may be assembled in a predetermined manner".
U.S. Pat. No. 5,534,074 to Koons is directed to a vertical boat for holding semiconductor wafers. The boat includes a plurality of rods having slots cut along their lengths. The configuration of the slots is intended to reduce shadowing on wafers placed within the boat during processing. The rods are cylindrical, and are specified as being made from fused quartz, although "any known material suitable for holding wafers may be used."
U.S. Pat. No. 4,872,554 to Quernemoen shows a reinforced carrier for silicon wafers and the like. The carrier includes side components consisting of tubular rails with wafer spacing and supporting teeth projecting therefrom. The rails are made from plastic, and may be provided with rigid inserts for stiffening purposes. The teeth can be integrally molded with, or fused to, the rails.
U.S. Pat. No. 5,752,609 to Kato et al. is directed to a wafer boat including a plurality of rods arranged to support ring members. A plurality of wafer supporting pieces are associated with the ring members, and include angular projections for contacting the wafers. The Kato et al. patent also illustrates a wafer boat including a plurality of cylindrical quartz rods having wafer support recesses formed therein.
The theoretical advantages provided by pure silicon structures are well known. Conventional towers and boats are typically made from quartz or silicon carbide, which introduce contamination and become unstable at higher temperatures. By fabricating wafer holding structures from the same materials as the wafers themselves, the possibility of contamination and deformation would be minimized. The structure would react to processing temperatures, conditions, and chemistry in exactly the same way that the wafers would, thus greatly enhancing the overall effective useful life of the structure.
Unfortunately, standard assembly of silicon structures in a "predetermined manner" as set forth in Tanaka et al. is one of the reasons that pure silicon has not gained wide acceptance as a material for structures such as boats and towers. The difficulties of working with monocrystalline and polycrystalline silicon have led to the development of structures such as that shown in Tanaka et al., wherein, when considering monocrystalline silicon as the material of choice, the connections between the support members and the end members are not described at all, and the only specifically described method of fabricating support structures involves cutting extruded tubular members. Such support structures are inherently less stable than those made from more traditional and easily-worked materials such as quartz or silicon carbide.
Similarly, the patents to Koons, Quernemoen, and Kato et al. fail to address the specific problems of providing a strong, reliable wafer support structure that reduces shadowing and contamination. The projections and slots described in these patents, while effective to some extent, are either not suited for fabrication from materials such as silicon, or require a relatively large cross-sectional area to provide stable and precise wafer support.
Silicon is perceived as being extremely fragile and difficult to weld. Due to these perceptions, known silicon structures are widely believed to be delicate at best, and unreliably flimsy at worst. Consequently, they have failed to receive broad commercial acceptance.
It can thus be seen that the need exists for a strong, reliable support member for wafer processing fixtures that will reduce shadowing and contamination while providing stable and precise wafer support.